As per section 2(35) read with section 24(1) of model GST law, where a tax invoice has been issued for supply of any goods and/or services and the taxable value and/or tax charged in that a tax invoice is found to exceed the taxable value and/or tax payable in respect of such supply, the taxable person, who has supplied such goods and/or services, may issue to the recipient a credit note containing prescribed particulars. (Refer Q. No. 24).

Accordingly,

1. credit note has to be issued by taxable person who had earlier issued a tax invoice for supply of any goods and/or services 
2. credit note has to be issued where tax invoice has charged excess value and/or excess tax charged than required,
3. credit note should contain prescribed particulars.

Thus, a credit note serves the purpose of accounting adjustment to settle the correct amount of value and tax.

Time Limit  for issue of Credit Note

As per section 24(1) of Model GST law, time of issue of credit note shall be, on or before the thirtieth day of September following the end of the financial year in which such supply was made, or the date of filing of the relevant annual return, whichever is earlier.

Accordingly, time of issue of credit note shall be-

1. on or before the thirtieth day of September following the end of the financial year in which such supply was made, or
2. the date of filing of the relevant annual return for the financial year,

    whichever is earlier.

It may be noted that annual return is required to be filed under section 30(2) on or before 31^st December of the financial year following the relevant financial year. In cases where such annual return is filed after 30^th September, the time limit for issuing credit note will be 30^th September only.

According to proviso to section 24(1), no credit note shall be issued by the taxable person if the incidence of tax and interest on such supply has been passed by supplier to recipient.

 Debit Note

As per section 2(36) read with section 24(2) of the model GST law, where tax invoice has been issued for supply of any goods and/or services and the taxable value and/or tax charged in that tax invoice is found to be less than the taxable value and/or tax payable in respect of such supply, the taxable person, who has supplied such goods and/or services, shall issue to the recipient, a debit note containing prescribed particulars.

Accordingly,

1. debit note has to be issued by taxable person who had earlier issued a tax invoice for supply of any goods and/or services, 
2. debit note has to be issued where tax invoice has charged short value and/or short tax charged than required,
3. debit note should contain prescribed particulars.

Thus, a debit note serves the purpose of accounting adjustment to settle the correct amount of value and tax.

Time Limit  for issue of Debit Note

As per section 24(2) of Model GST law, time of issue of debit note shall be, on or  before the thirtieth day of September following the end of the financial year in which such supply was made, or the date of filing of the relevant annual return, whichever is earlier.

 Accordingly, time of issue of debit note shall be,

1. on or before the thirtieth day of September following the end of the financial year in which such supply was made, or
2. the date of filing of the relevant annual return for the financial year,

            whichever is earlier.

It may be noted that annual return is required to be filed under section 30(2) on or before 31^st December of the following financial year. In cases where such annual return is filed after 30^th September, the time limit for issuing debit note will be 30^th September only.